1. Field of the Invention
The present invention relates to a method of flattening or smoothing out an irregular surface of an insulation layer by ion etching in the fabrication of an electronic device such as a thin-film magnetic head, a semiconductor device, or the like.
2. Description of the Prior Art
Thin-film magnetic heads of a laminated structure include a substrate, a lower magnetic layer deposited on the substrate, a first insulation layer deposited on the lower magnetic layer, a single or a plurality of coil conductor layers deposited on the first insulation layer, a second insulation layer deposited on the coil conductor layer(s), and an upper magnetic layer deposited on the second insulation layer. The second insulation layer, made of SiO.sub.2 for example, is deposited by sputtering or evaporation on the coil conductor layer(s) before the upper magnetic layer is deposited thereon. The second insulation layer thus deposited however has surface irregularities because the coil conductor layer(s) underneath the second insulation layer has surface irregularities. Therefore, a magnetic material such as Sendust or an amorphous material, if directly deposited as the upper magnetic layer on the second insulation layer by sputtering or evaporation would also have corresponding surface irregularities and hence would be subject to a reduction in magnetic permeability, with the result that the recording and reproducing efficiency of the produced magnetic head would be reduced. To eliminate the above drawbacks, the uneven surface of the second insulation layer is normally flattened or smoothed out.
In one conventional process of smoothing out surface irregularities, a photoresist is coated on an insulation layer of SiO.sub.2 having surface irregularities to a thickness of about 8 micrometers, thereby smoothing out the surface irregularities of the insulation layer. Then, the photoresist is heated to at least 130.degree. C. to improve the surface flatness thereof. Thereafter, the photoresist and the insulation layer are etched by an ion beam applied at such an angle that they can be etched at the same etching rate, the angle being referred to as an angle of ion bombardment formed between the ion beam and a line normal to the surface being etched. Such an ion etching process is known as ion milling or ion machining. After the insulation layer has been smoothed out, an upper magnetic layer of Sendust or an amorphous material is sputtered or evaporated as an upper magnetic pole on the insulation layer. Since the upper magnetic layer on the flat insulation layer is substantially free of surface irregularities, its magnetic permeability is not lowered and the resultant thin-film magnetic head has an improved recording and reproducing efficiency.
However, the above prior flattening process has the following difficulties: Where the ion milling is carried out using an inert gas such as Ar, the angle of ion bombardment at which the photoresist and the insulation layer can be etched at the same rate is about 75.degree. which is larger than the angle of ion bombardment (about 55.degree.) at which the photoresist can be etched at a maximum etching rate. When the photoresist is etched at such a large angle of ion bombardment, the etched photoresist material tends to stick to minute impurities on the surface of the photoresist, or is apt to stick to or be polymerized on the surface of the photoresist due to diffusion, with the consequence that there are formed projections during the etching process. As the insulation layer is etched under such a condition, the projections of the photoresist are transferred to the insulation layer since the photoresist and the insulation layer are etched at the same rate. Therefore, the insulation layer to be smoothed out also has many projections (indicated at 100A, 100B in FIG. 5 of the accompanying drawings) formed on its surface. The magnetic layer of Sendust or an amorphous material sputtered or evaporated on the insulation layer has corresponding projections, and the recording and reproducing efficiency of the produced thin-film magnetic head is low.
Another problem is that the surface irregularities of the photoresist vary dependent on the pattern size (pattern width and pattern-to-pattern distance) of the coil conductor layer(s). FIG. 6 of the accompanying drawings illustrates how the height a of surface irregularities of the photoresist coated on the insulation layer to a thickness of about 8 micrometers varies with the pattern width W and the pattern-to-pattern distance W on the insulation layer, the surface irregularities of the insulation layer having a height of 2 micrometers. As is apparent from FIG. 6, as the pattern size becomes larger, the flatness of the photoresist is reduced, and an appreciable amount of surface irregularities remains on the surface of the etched insulation layer.